1. Field of the Invention
The invention relates to a polycrystalline silicon rod pair and to a method of producing polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as the starting material in the production of monocrystalline silicon by crucible pulling (Czochralski or “CZ” method) or by zone melting (float zone or “FZ” method). This monocrystalline silicon is cut into wafers and, after a great many mechanical, chemical and mechanochemical processing operations, is employed in the semiconductor industry for fabricating electronic components (chips).
However, in particular, polycrystalline silicon is needed to a greater extent for producing mono- or multicrystalline silicon by pulling or casting methods, this mono- or multicrystalline silicon being used for fabricating solar cells for photovoltaic applications.
The polycrystalline silicon is typically produced by the Siemens process. This comprises heating slim rods of silicon in a bell-shaped reactor (known as a “Siemens reactor”) to surface temperatures of 900-1200° C. by direct passage of current, and introducing a reaction gas comprising a silicon-containing component, in particular a halosilane, and hydrogen via inlet nozzles. These halosilanes decompose at the surface of the slim rods. This causes elemental silicon from the gas phase to be deposited onto the slim rods.
The silicon rods are held in the reactor by special electrodes generally made of high-purity electrographite. In each case two slim rods having different voltage polarities on the electrode holders are connected by a bridge at the other slim rod end to form a closed electrical circuit. Electrical energy for heating the slim rods is supplied via the electrodes and their electrode holders.
The diameter of the slim rods increases during the deposition. The electrode simultaneously grows into the rod base of the silicon rods, starting at its tip.
The employed material of construction of the electrodes is generally graphite since graphite is available in very high purity and is chemically inert under deposition conditions. Graphite further has a very low specific electrical resistance.
Once a desired target diameter for the silicon rods has been achieved, the deposition process is terminated and the glowing silicon rods are cooled down and deinstalled.
Subsequently, the obtained U-shaped rod pairs made of polysilicon are typically cut to length at the electrode and bridge ends and comminuted into chunks. Comminution is carried out using a crusher, for example a jaw crusher. Such a crusher is described in EP 338 682 A2, for example. This is optionally preceded by precomminution using a hammer. The graphite electrode is typically removed beforehand.
US 20120175613 A1 discloses a method of producing a polycrystalline silicon piece consisting of a CVD process for producing a polycrystalline silicon rod by depositing silicon onto a filament wire, of which one end is attached to a first electrode and the other is attached to a second electrode, a process for removing the polycrystalline silicon rod from the reactor and a comminution process for comminuting the silicon rod into silicon pieces which comprises removing at least 70 mm from the electrode end of the polycrystalline silicon rod (base shortening process) prior to the comminution process. A preferred embodiment comprises covering the surface of the polycrystalline silicon rod with a bag-like member made of polyethylene prior to removal of the rod from the reactor.
DE 10 2013 206 339 A1 discloses a method of deinstalling polycrystalline silicon rods from a reactor, wherein the reactor comprises U-shaped rod pairs, wherein one of the U-shaped rod pairs is completely enveloped by a body having an outside wall and an inside wall, and the body, together with the rod pair enveloped by it, is removed from the reactor using a crane, a winch or a grab. The body may have an inside wall made of steel and the rod pair is covered with a plastics material bag before it is enveloped by the body.
The plastics material bags employed in the two abovementioned methods are intended to protect the polycrystalline silicon rod from contamination.
However, it was found that using plastics material bags made of PE film having a thickness of 150 μm or less can result in perforations in the plastics material bags when the bags are pulled over the polycrystalline silicon rods. Experience has shown that up to 50% of the plastics material bags employed exhibit perforations. The contamination of the polycrystalline silicon rods with foreign particles has proven particularly problematic. The origin of the foreign particles can predominantly be traced back to the destruction of the plastics material bags and film residues formed.